1. Field of the Invention
The present invention relates to an optical-fiber characteristics measuring apparatus which measures various characteristics of an optical fiber by emitting an optical pulse to the optical fiber and performing an optical heterodyne detection involving the combination of returned light from the optical fiber with local oscillation light.
2. Description of the Related Art
FIG. 3 is a block diagram illustrating the structure of an optical-fiber characteristics measuring apparatus according to the related art. The operation of this optical-fiber characteristics measuring apparatus will be described below. When a light source 31 emits coherent light 31a of a frequency f0 to an optical directional coupler 32, the coherent light 31a passes through the optical directional coupler 32 and enters an optical pulse generator 33 as coherent light 32a. The optical pulse generator 33 converts this coherent light 32a into pulse light 33a. It is to be noted that the coherent light 32a and the pulse light 33a have the same frequency as the frequency f0 of the coherent light 31a. 
Next, an optical frequency converter 34 performs frequency conversion by shifting the frequency of the pulse light 33a by a predetermined frequency xcex94f and sends out coherent light 34a having a frequency xe2x80x9cf0+xcex94fxe2x80x9d. This pulse light 34a travels through an optical amplifier 35, an optical switch 36 and an optical connector 37 and is emitted as pulse light 37a toward an optical fiber 38 to be measured. When this pulse light 37a enters the to-be-measured optical fiber 38, reflection or scattering respectively produces reflected light or scattered light in accordance with the state in the to-be-measured optical fiber 38. Part of the reflected light or scattered light travels as returned light 38a through the optical connector 37 and the optical switch 36. Then, returned light 36b is emitted toward the balanced-light reception circuit 40.
The balanced-light reception circuit 40 converts the returned light 36b into an electric signal through balanced-light reception with the coherent light 32b of the frequency f0 emitted from the optical directional coupler 32. Specifically, an optical directional coupler 41 combines the coherent light 32b and the returned light 36b, and a photoelectric converter 42 converts the combined optical signal into an electric signal which is in turn amplified by an electric signal 43a by an amplifier section 43. This electric signal 43a is input to a signal processing section 46 through a low-pass filter 44 and an amplifier section 45. The signal processing section 46 acquires various characteristics of the to-be-measured optical fiber 38 based on the input electric signal and processes this electric signal on the time axis to prepare the distribution on the distance axis of the to-be-measured optical fiber 38.
According to the conventional optical-fiber characteristics measuring apparatus, as apparent from the above, the optical scheme using the optical frequency converter 34 shifts the frequency of the pulse light 37a to be input to the to-be-measured optical fiber 38 by the predetermined frequency xcex94f with respect to the frequency of the coherent light 31a. Then, the local oscillation light (coherent light 32b) and the returned light 36b are combined, yielding a beat signal. The frequency xcex94f is set in accordance with the frequency of the returned light 36b in such a way that the frequency of the beat signal (i.e., the difference between the frequencies of the local oscillation light and the returned light) lies in an electrically processable range. Accordingly, backward scattered light, such as the Rayleigh scattered light and Brillouin scattered light, and reflected light, which is produced in the to-be-measured optical fiber 38, can be detected as returned light.
The use of such an optical frequency conversion scheme requires that the optical frequency converter 34 should be constituted by an optical frequency shifter or by an optical ring comprising several optical components. This complicates the structure of the optical-fiber characteristics measuring apparatus. When an optical ring system is used, for example, while pulse light is travels along the optical ring, new pulse light cannot be input to the optical ring. This restricts the cycle period of the pulse light that is emitted from the optical ring, thus disabling fast measuring of the characteristics of the to-be-measured optical fiber. In addition, the frequency conversion increases the frequency of the pulse light, thereby restricting the pulse width of the pulse light.
Accordingly, it is an object of the present invention to provide an optical-fiber characteristics measuring apparatus that has a simple structure which does not require frequency conversion of pulse light to be input an optical fiber to be measured and does not restrict the cycle period of the pulse light, thereby ensuring fast measuring of the characteristics of the optical fiber using a fast optical output.
To achieve the above object, according to one aspect of this invention, there is provided an optical-fiber characteristics measuring apparatus for converting coherent light into pulse light, emitting the pulse light to an optical fiber, converting an optical signal acquired by balanced-light reception of returned light from the optical fiber and the coherent light into a first electric signal, and obtaining characteristics of the optical fiber from a frequency component of the returned light included in the first electric signal, which apparatus comprises signal generation means for generating a second electric signal having a frequency approximately coincident with a frequency of an optical signal to be detected in those optical signals included in the returned light; and mixing means for mixing the first electric signal and the second electric signal to thereby detect a frequency component of the optical signal to be detected.
According to this invention, as specifically described above, the frequency component of the desired optical signal is detected by producing a first electric signal by conversion of the optical signal that is acquired by the balanced-light reception of returned light and coherent light, producing a second electric signal whose frequency approximately matches with the frequency of the optical signal to be detected of optical signals included in returned light, and then mixing the first and second electric signals together. In the case of detecting the returned light by using a beat signal obtained by combining the returned light and local oscillation light (coherent light), therefore, the frequency component of the optical signal included in the returned light can be detected even if the frequency band of the signal processor for acquiring the characteristics of an optical fiber is not matched with the frequency component of the beat signal. This can ensure excellent coherent detection according to the frequency component of reflected light or any of various kinds of scattered lights contained in the returned light. Further, it is unnecessary to shift the frequency of the pulse light to be sent to an optical fiber, thus eliminating the need for a circuit, such as an optical frequency shifter or an optical ring system. This can help make the structure of the optical-fiber characteristics measuring apparatus simpler. Furthermore, there is no restriction on the cycle period of pulse light, so that the pulse light can be emitted in a shorter period, thereby ensuring fast measuring of the characteristics of the optical fiber.
In this optical-fiber characteristics measuring apparatus, the signal generation means may detect a spectrum of the optical signal to be detected by changing the frequency of the second electric signal over a spectrum width of the optical signal to be detected.
In this case, the spectrum of the optical signal to be detected is detected by changing the frequency of the second electric signal over the spectrum width of the to-be-detected optical signal. Therefore, even if the spectrum width of an optical signal contained in the returned light is wider than the spectrum width of the second electric signal as in the case of scattered light, therefore, the spectrum of every optical signal contained in the returned light can be detected.
Furthermore, in this case or in the optical-fiber characteristics measuring apparatus of the above aspect, the signal generation means may set the frequency of the second electric signal in accordance with a type of the optical signal to be detected.